Conventionally, as a catalyst for purifying the exhaust gases of automobiles, three-way catalysts have been used, three-way catalysts which carry out the oxidation of CO and HC in the exhaust gases and the reduction of NOx therein simultaneously at the theoretical air-fuel ratio (stoichiometry), thereby purifying them. As for such a three-way catalyst, the following has been known widely, for instance: a coating layer is formed from a porous oxide, such as γ-alumina, on a heat-resistant substrate comprising cordierite, and the like, and then a noble metal, such as platinum (Pt) and rhodium (Rh), is loaded on the coating layer.
This three-way catalyst functions as an oxidizing catalyst mainly in lean atmosphere; and functions as a reducing catalyst as well in atmospheres from stoichiomeric to rich atmosphere, and can thereby purify NOx by reducing them by means of CO and HC in the exhaust gases.
By the way, although the loaded noble metal demonstrates the catalytic actions in the three-way catalyst, since the temperature region over which the catalytic actions of the noble metal are demonstrated lies in high-temperature region comparatively, there has been such a problem that it is less likely to purify the harmful components in low-temperature region. Accordingly, when the exhaust gases fall in low-temperature region in winter, at the time of start up, and the like, there has been such a drawback that the purifying activities are low.
Hence, it has been carried out to load the noble metal with high density onto the upstream portion to which the exhaust gases flow in. By means of thus loading the noble metal with high density onto the upstream portion, the probability of the noble metal contacting with the exhaust gases enhances, and thereby the probability of the oxidation reactions of CO and HC taking place enhances. And, when the oxidation reactions take place once, the ignition propagates so that the oxidation reactions proceed furthermore. Moreover, since the oxidation reactions are exothermic reactions, there is also such an action that the three-way catalyst is heated by means of the reaction heats to undergo temperature increment so that it undergoes temperature increment to the activation-temperature region of the noble metal quickly. Therefore, when loading the noble metal with high density onto the upstream portion, the purifying activities in low-temperature region improve by means of the synergic effect of these.
For example, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-181,374, there is a description on loading a noble metal more onto an upstream-side catalyst than onto a downstream-side catalyst in a tandem catalyst in which the two catalysts are put in position in series. Moreover, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-252,565, there is a description on a catalyst in which an upper-layer coating layer is further formed on a surface of a coating layer's upstream portion to which exhaust gases flow in. In accordance with this catalyst, it is possible to make the loading density of the noble metal on the upstream portion higher by means of the noble metal that is loaded on the upper-layer coating layer.
However, when simply making the loading density of the noble metal higher, there is such a problem that the absolute amount of the noble metal increases so that the cost has risen. Moreover, when making the loading density of the upstream-side noble metal higher, the HC purifying performance in low-temperature region improves. However, even when making the loading density of the upstream-side noble metal higher, it hardly affects the purifying performance after the temperature of the catalyst rises up to the activation temperature of the noble metal (hereinafter, referred to as “after warming up”), and accordingly the noble metal has been wasted. Moreover, there is even such a problem that the NOx purifying performance after warming up is not sufficient.
Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-181,374; and
Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-252,565